Bleaching of cellulose pulp in a first chlorine dioxide bleaching step

ABSTRACT

The method is for bleaching cellulose pulp having an MC consistency in a first step with chlorine dioxide in a bleaching sequence with several bleaching steps. The pulp is bleached in this first chlorine dioxide step in a first phase at a lower temperature of 60±10° C. during a retention time of from 30 seconds to 30 minutes, after which the pulp is heated by at least 10° C. to a reaction temperature of 90±10° C. for a second phase, which has a retention time of 60-200 minutes, With this process modification of a first chlorine dioxide step, an efficient destruction of undesired organic acids is obtained, while at the same time improved viscosity/strength of the pulp can be obtained of the finally bleached pulp with a whiteness of approximately 90% ISO.

The present invention concerns a method for bleaching cellulose pulp in a first chlorine dioxide step.

THE PRIOR ART

In association with the bleaching of cellulose pulp with chlorine dioxide, sequences of several steps have most often been used, with two or more bleaching steps in which chlorine dioxide is added batchwise. A typical bleaching sequence of several steps can be: D₀(EO)D₁D₂, D₀(EOP)D₁D₂ or solely D₀(EOP)D₁ with washes between the steps, where the first chlorine dioxide step is equivalent to the D₀-step and where the bleaching sequence is normally preceded by oxygen gas delignification, O₂. Other bleaching agents can sometimes be used, such as ozone, Z, which can be batchwise added before or after D₀, either with or without an intermediate wash. A step using pure peroxide, P, may be used as a final bleaching step.

U.S. Pat. No. 3,745,065 reveals a chlorine dioxide bleaching stage for cellulose pulp with a tower with upward flow and an immediately subsequent tower with downward flow. The process is controlled in this case such that the pulp is processed in these towers at a temperature in the range 60-85° C. (140-185° F.) and where the pulp has a retention time in the tower with upward flow of 25 minutes and a retention time in the tower with downward flow of 2-3 hours. U.S. Pat. No. 3,745,065 concerns, more specifically, how the addition of bleaching chemicals is controlled by measuring residual gases at the highest point at the transition between the tower with upward flow and that with downward flow, where the pressure is lowest and gases are expelled. It is here specified that the tower with downward flow is normally completely filled, with a pulp level just below the top of the tower with downward flow.

Kvaerner Pulping AB (previously KamyrAB) marketed during the 1980s special bleaching towers in which the tower with upward flow was arranged concentrically within a larger surrounding tower of downward flow. The pulp was thus fed into the bottom of the tower of upward flow (in actuality a coarse pulp pipe), after which the pulp flowed out of the tower of upward flow, which opened just under the top of the larger surrounding tower of downward flow, in order subsequently to sink downwards to an outlet at the bottom of the tower of downward flow. This tower design allowed a compact bleaching step to be achieved and simultaneously allowed heat from the tower of upward flow to radiate out into the tower of downward flow.

WO;A;01/96656 (with date of priority 13 Jun., 2000) reveals that the known combination of tower of upward flow and tower of downward flow in one chlorine dioxide step can be adapted to different cellulose pulps. It is specified here that when the chlorine dioxide bleaching step is adapted to softwood (SW) pulp, the retention time can be longer, specified as 2-4 hours, than the retention time can be for hardwood (HW) pulp, specified as 10 minutes-2.5 hours, although in certain cases they may be identical (overlapping). With respect to the temperature, this may be lower for softwood pulps, 50-70° C., and for hardwood pulps 60-85° C., but it thus is not necessarily lower for softwood pulps. It is specified in this adaptation strategy that the conditions specified are also to be suitable for a first, second or third chlorine dioxide step (the D₀-, D₁- and D₂-steps, respectively).

Selective adjustment of the temperature between the phases in the tower of upward flow and the tower of downward flow is not used in the bleaching step in any of these solutions according to the prior art.

It has become apparent that the bleaching and delignification process in a D₀ step in which the input cellulose pulp has a high level of residual lignin, typically having a kappa value of 7 and often having a kappa value greater than 10, is significantly different from the bleaching processes that takes place in the subsequent D₁ and D₂ steps in a multistep sequence, in which the level of residual lignin in the input pulp to D₁ is very low, typically having a kappa value that lies well under 6.

In the same manner, there is normally present a high level of organic acids, which gives a poor light-stability for the finally bleached pulp, in the cellulose pulp when bleaching hardwood that has been produced according to the sulphate cooking process (either in batches or in continuous cooking, conventionally followed by oxygen gas delignification after the cooking) but before the first chlorine dioxide step. It is desired that these organic acids should be selectively removed from, in particular, hardwood pulps, and this is most efficiently carried out using a high-temperature treatment under acid conditions during a long period, typically 1-3 hours. E. Venemark revealed this technique as early as 1963 in SE,C,225253 (equivalent to GBI.062.734).

A combination of a conventional D₀-step with subsequent acid treatment at a considerably raised temperature and with a long retention time is described in D. Lachenal & C. Chirat, 1998 Pulping Conference, “High temperature chlorine dioxide delignification: A breakthrough in ECF bleaching of hardwood kraft pulps”, pages 601-604. However, a conventional D₀-step with a long retention time is used in this case.

AIM AND PURPOSE OF THE INVENTION

It has surprisingly become apparent that in a first chlorine dioxide step, often denoted a D₀-step, this first D₀-step is in a first phase to be carried out at a conventional low temperature of 60±10° C. with very short retention time, and then subsequently to be carried out in a second phase at a considerably increased temperature 90±10° C. and with a long retention time. This method allows an intrinsic viscosity improved by 20-30 units (measured in dm³/kg), relative to a modified first D₀-step at raised temperature that has been adapted in order to destroy the undesired inorganic acids to be obtained.

The term “a first chlorine dioxide step” also known as a D₀ step, will subsequently be taken to refer to a bleaching step in which a chlorine multiple of at least 1.0 is used, i.e. at least 1 kg of active chlorine added batchwise for each kappa unit of the input pulp, or, alternatively, that a total amount of active chlorine added in batches that exceeds 10 kg/BDMT (Bone Dry Metric Tonne) of pulp, and where no preceding step with the batchwise addition of active chlorine in excess of these levels is used in the bleaching sequence.

The principal aim of the invention is to achieve an improved D₀-step in which the highest possible pulp strength is obtained while at the same time the pulp is readily bleached to the desired final whiteness in subsequent bleaching steps.

DESCRIPTION OF DRAWINGS

FIG. 1 shows suitable process equipment in which hardwood pulp is bleached with chlorine dioxide according to the invention;

FIG. 2 shows the improved viscosity that is obtained with a short, cold first phase at 60° C. for 3, 6, or 12 minutes, before a step lasting 120 minutes at 90° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Process equipment is shown in FIG. 1 in which the method according to the invention can be applied. Bleaching of cellulose pulp here takes place at a mean consistency of 7-22%, preferably between 10-14%, in a first bleaching step with chlorine dioxide. The pulp 1 is fed in from a preceding sulphate cooking step, and often from a preceding oxygen gas delignification stage, to a pulp chute 2. The pulp for the bleaching stage has an input kappa value that exceeds 7.0. Heating can take places in the pulp chute using steam ST₁. A pulp level is established in the pulp chute, and an MC pump 3 is arranged at the bottom of the chute. The pulp is pressurised by the pump 3 to an excess pressure of at least 2 bar (2 atmospheres), preferably with an excess pressure of 4-8 bar.

The pulp is subsequently led to a second heating means 4 and a mixer 5 placed in series, where chlorine dioxide ClO₂ is mixed into the pulp using the mixer once the pulp has been given the correct temperature in the heating means.

The temperature of the pulp at this position is to be maintained at a low level, in the range 60±10° C.

The pulp is subsequently led to the reactor 6 at the established reaction temperature, which reactor comprises a tower of upward flow 6 a and a tower of downward flow 6 b connected in series after each other. The tower of upward flow 6 a is in this case a coarse pulp pipeline which gives a retention time/ascension time in the tower of upward flow of 1-30 minutes during normal production.

The pulp is, according to the invention, to be bleached in a first phase in the tower of upward flow at low temperature during a retention time that is shorter than 30 minutes, and directly after this first phase is to be heated by at least 10° C. without an intermediate wash, after which the pulp is given a retention time at this raised temperature in a second step in the range 60-200 minutes, preferably 100-180 minutes and typically approximately 120 minutes. Therefore, the pulp can, as is shown in the drawing, be heated in the transition between the tower of upward flow 6 a and the tower of downward flow 6 b, with at least one heating means, preferably a steam mixer 12. It is appropriate that the retention time during the first phase does not exceed 20 minutes and it is even more preferable that it does not exceed 10 minutes. It is, however, necessary that a certain minimum reaction time is established for this first phase, and this is why the retention time in the first phase should amount to at least 30 seconds and preferably at least 1 minute. This minimum retention time provides a clear distinction from any batchwise addition of chlorine dioxide in which the heating may take place immediately after the mixing in of the chlorine dioxide. It is conventional, however, that the “correct” or the desired reaction temperature is usually established before the chlorine dioxide is mixed into the pulp.

For retention times of 1-3 minutes, the first phase can be established in a pipeline of a few tens of meters, which pipeline in the form of a U-bend can be located in a single horizontal plane, or in a vertical plane, possibly followed by a second phase at an increased temperature in a reactor of upward flow. The increase in temperature obtained with the heater/mixer 12 before the second phase is in the range 10-30° C., which gives a starting temperature for the second step in the range 90±10° C.

The tower of downward flow has a volume that, when hydraulically filled with pulp, gives a retention time of 60-200 minutes during normal production. The retention time of the pulp in the tower of downward flow in the reactor can be controlled by adjusting the level in the tower of downward flow to the desired level by the level sensor, the differential pressure-sensor, which is connected to the output pump 9. A rotating scraper 8 is located at the bottom of the tower of downward flow 6 b in order to facilitate output from the tower.

The pulp is led, after pumping out, through the pump 9 to a wash press, in which the pulp is washed with cleaner process water PW. This process water can preferably, in closed bleaching plant in which the process water is led in a countercurrent flow relative to the flow of pulp, be a filtrate from the subsequent washing steps (not shown in the drawing).

It is appropriate that the second heating means 4 and the heating means 12 can be constituted by at least one static (shown in the drawing) or dynamic steam mixer, through which pulp flows during the addition of steam directly to the pulp. A slit mixer with locally reduced area of pulp flow at the addition of steam can be used in a static mixer, or a conventional static mixer in which the steam is added to a plug flow in the pulp pipeline. A dynamic mixer may be of the type equivalent to a simple wisp mixer, or it may be a multizone slit mixer of a type equivalent to a DUALDMIX (previously known as an MC-mixer).

The second heating means 4 and the heating means 12 can, as is shown in the drawing, be supplemented with, or be solely constituted by, a fluid mixer through which pulp flows during the addition of pressurised superheated process fluid through a pump 30. A somewhat higher degree of dilution of the pulp concentration occurs when using such heating. This process fluid can preferably be filtered WF from a washing arrangement 10 arranged to wash the pulp following the chlorine dioxide step, which filtrate has been heated to the desired temperature of the process fluid by steam ST₂ in an indirect heat exchanger 31.

FIG. 2 reveals the surprising result of experiments. The viscosity of finally bleached pulp from a D*(EOP)D sequence, a hot D₀-step (D*=120 minutes, 90° C.) is here compared with different combinations of a short colder phase before the hotter final phase in the D₀-step, with retention times of 3, 6 and 12 minutes in the colder phase.

The initial pulp for the bleaching in the experiments has been hardwood pulp with a kappa value of 11.4, and the (EOP)-step has been maintained identical, with the same consumption of peroxide, 3 kg/BDMT, the same batch of alkali, 15 kg/BDMT, and the same final pH, 11.0.

The noticeable improvement in the viscosity is remarkable, particularly for the short steps of 3 and 6 minutes, which give a viscosity for a pulp with a final whiteness of ISO 90 that is about 25 units better (907 vs. 932). An improvement is also obtained with a colder phase of 12 minutes.

In this series of experiments, 15.9 kg of ClO₂ has been used in a first D-step (D* in the D(OP)D sequence), calculated as active chlorine per tonne of pulp (ADMT), and the total batchwise addition of ClO₂ calculated as active chlorine per tonne of pulp (ADMT) in both of the chlorine dioxide steps amounted to 23, 31 and 38 kg for final whitenesses of approximately 89% ISO, 90% ISO and 91% ISO, respectively.

It is possible through this modification of the D₀-step to compensate to a significant degree for the loss of strength that otherwise results from carrying out the D₀-step at a higher temperature (90±10° C.), in comparison with a conventional D₀-step at lower temperature (60±10° C.). This loss of strength at the D₀-step at high temperature typically lies within the range 5-10% of the viscosity of the finally bleached pulp.

And it is possible to completely avoid the loss of strength of the pulp that is experienced by storing the pulp in a storage tower under acid conditions, for a long time and at a high temperature, a procedure that is otherwise applied in order to destroy the unwanted organic acids. 

1. A method for bleaching cellulose pulp, comprising: providing a first chlorine dioxide bleaching step in a bleaching sequence where cellulose pulp has an initial kappa value for the first chlorine dioxide bleaching step that exceeds 7, bleaching the cellulose pulp at medium consistency, heating the cellulose pulp to a primary temperature in a range of 50-70° C., mixing chlorine dioxide into the cellulose pulp, establishing the cellulose pulp at the primary temperature leading the cellulose pulp to a first phase, and the cellulose pulp being bleached in the first chlorine dioxide bleaching step for a first retention time that docs not exceed 20 minutes, and is directly after the first chlorine dioxide bleaching step, without an intermediate wash, heated by at least 10° C., after which the cellulose pulp is given a second retention time in a second step in a range of 60-200 minutes.
 2. The method according to claim 1 wherein the first chlorine dioxide bleaching step does not exceed 10 minutes.
 3. The method according to claim 1 wherein the first retention time is at least 30 seconds.
 4. The method according to claim 1, wherein a temperature is increased in a range of 10-30° C. before the second step to provide an initial temperature for the second step in a range of 80-100° C. 